Chemical recycling process of polybutylene succinate and biopolyester obtained by said process

Abstract

The present invention relates to a chemical recycling process for polybutylene succinate biopolyester (PBS) by means of a sequential depolymerization and repolymerization process. In the repolymerization stage, the self-polycondensation of the bis(4-hydroxybutyl) succinate (BSB) obtained from the depolymerization is carried out in the absence of solvents, thus avoiding the prior esterification step and introducing only the BSB into a reactor. Advantageously, the repolymerization stage incorporates the catalyst titanium(IV) tetraisopropoxide (TTiP), which, when mixed with the BSB under appropriate conditions, allows for obtaining a PBS with a weight molecular weight (MW) between 50,000 and 250,000 g / mol. The invention enables the recycling of PBS, used primarily in food packaging applications, yielding a product with characteristics similar to the original, said PBS also being the subject of the invention.

Description

Chemical recycling process of polybutylene succinate and biopolyester obtained by said process FIELD OF INVENTION The present invention consists of the chemical recycling of polybutylene succinate biopolyester (PBS), of petrochemical origin or obtained from partially or totally renewable sources, and the subsequent application of a chemical recycling process to be able to recover it after its use and reuse it indefinitely. The PBS recycling process of the invention consists of depolymerization and subsequently, and necessarily, repolymerization using titanium tetraisopropoxide (IV) (TTiP) as a catalyst to obtain PBS with a molecular weight between 50,000 and 250,000 g / mol. Therefore, the invention falls within the application of the principles of Bioeconomy, since it makes use of materials developed from natural and renewable resources, and of the Circular Economy, since the PBS is completely recycled to be used again successively. BACKGROUND There are known procedures related to the physical or chemical recycling of biopolymers, several of which are found on the polylactic polymer (PLA) or mixtures. On the other hand, the depolymerization process by solvolysis using PBS together with 1, 4-butanediol (1, 4-BDO) as a nucleophile, TTiP as a catalyst and obtaining as a product the compound bis (4-hydroxybutyl) succinate (BSB) is disclosed in patent document WO2022214640A1, which refers to the chemical recycling of polymers such as polyolefins, or compounds such as polyester that have properties similar to polyolefins. Other patent documents, such as CN115968385A, refer to monomers and ligamers for use in the depolymerization of bioplastics, particularly biodegradable compositions comprising polyhydroxyalkanoates and polyesters. The monomers obtained include, among others, 1,2-ethanediol, 1,3-propanediol, and 1,4-butanediol. Document KR102452867B1 discloses a transesterification reaction for producing high value-added derivatives using the functional group attached to DMT. For the depolymerization process, at least one of the following is used: alkali metal carbonate, alkali hydroxide, alkali metal alkoxide, alkaline earth metal oxide, and guanidine-based organic compounds in the transesterification reaction using DMT and monohydric and / or polyhydric alcohol as raw materials. WO2024005445A1 discloses a method for the depolymerization of polymers containing an ester group, specifically an aromatic compound with an alkoxy functional group and a compound with an alcohol group, which are mixed and used as a reaction solvent to remove contaminants. In the depolymerization method, the alcohol is used as a co-solvent to facilitate nucleophilic attack. However, none of the disclosures found propose a chemical recycling of PBS for indefinite use by including a subsequent repolymerization stage - using TTiP as a catalyst - to obtain PBS with a molecular weight between 50,000 and 250,000 g / mol, an invention now proposed that contributes to the development of solutions to reduce the environmental impact of plastics within the framework of the Circular Economy. DESCRIPTION OF THE INVENTION The present invention relates to a chemical recycling process which consists of two necessarily consecutive stages or processes that correspond to a first depolymerization, followed by a repolymerization which uses as raw material the product obtained in the depolymerization. It should be noted that PBS is currently commercially available, either of petrochemical or partially renewable origin, or has been developed on laboratory scale in a fully renewable format. Following the known use of PBS, with food packaging being one of its main applications, the starting material is obtained that will be used in the chemical recycling process of the present invention. According to the essence of the invention, the claimed chemical recycling process uses PBS from known uses as raw material, and said process consists of a depolymerization process and a repolymerization process, carried out consecutively. Thus, depolymerization comprises the following stages: PBS solvolysis reaction in a temperature-controlled reactor with agitation, a cooling system, and pressure, where PBS is introduced along with 1,4-BDO and heated between 130°C and 200°C (optimum 170°C), yielding the compound BSB. It should be noted that, optionally, the solvolysis reaction can be carried out with the catalyst TTiP. Whereas in repolymerization the self-polycondensation of the BSB is carried out, in the absence of solvents, avoiding the esterification stage and introducing only the BSB obtained from depolymerization, into a reactor with temperature control, agitation, a cooling system and high vacuum, so that repolymerization comprises the following stages: - Introduction of the BSB obtained from the solvolysis reaction into the reactor, reactor closure, initial stirring at at least 25 rpm, and purging with an inert gas stream to reduce system humidity. The inert gas used is nitrogen or argon. - Heating of the reactor to a temperature equal to or greater than 100°C. - Addition to the reactor of TTiP catalyst with a concentration between 1803 and 2404 ppm, forming a mixture, and cessation of the inert gas stream input. - Application of vacuum in the reactor up to a pressure of at least 200 mbar. Preferably, the pressure applied in the reactor is less than 2 mbar. - Heating the reactor to a temperature of at least 230°C, with an application of a heating ramp of 0.5 to 25°C / min, taking care to avoid excessive bubble formation. - Gradually decreasing the reactor pressure until it reaches less than 1 mbar, preventing the sublimation of the BSB species formed (diester, oligomers, etc.). - Increase the initial stirring to at least 75 rpm, and maintain the mixture in the reactor until the mixture experiences an increase in viscosity and formation of PBS with a molecular weight in the range of 50,000-250,000 g / mol. - Cooling to room temperature, resulting in the solidification of the PBS. It should be noted that, optionally, prior to depolymerization, the PBS to be recycled is milled in an analytical mill, washed with water and / or ethanol under agitation for at least four hours at a temperature of 55°C, and vacuum dried at a temperature of at least 60°C for at least eight hours. After this, the PBS is ready for chemical recycling through the depolymerization and repolymerization processes described above. Alternatively, the solidified PBS obtained after cooling is milled in an analytical mill, washed, and dried. Preferably, the washing of the solidified PBS (which corresponds to a whitish solid mass) is carried out with water and / or ethanol under agitation for at least four hours at 55°C, and vacuum drying takes place at a temperature of at least 60°C for at least eight hours. The present invention also relates to the biopolyester PBS obtained (recycled PBS) which, as previously stated, has a molecular weight between 50,000 and 250,000 g / mol, i.e., similar to the commercial one, and preferably has a polydispersity index (ΐ) between 2 and 3. Thus, the PBS obtained from the recycling process of the present invention constitutes a product ready for reuse, promoting the Circular Economy. BRIEF DESCRIPTION OF THE FIGURES To complement the description that follows and to aid in a better understanding of the characteristics of the invention, according to a preferred embodiment thereof, figures are included as an integral part of said description, in which, for illustrative and non-limiting purposes, the following has been represented: Figure 1 shows a schematic of the depolymerization process of the chemical recycling of PBS according to a preferred embodiment of the invention. Figure 2 shows the characterization of the final product of depolymerization by proton nuclear magnetic resonance (1H-NMR) in deuterated chloroform, where the abscissa axis represents the chemical shift in parts per million (ppm). Figure 3 shows the reactor used for the repolymerization of PBS from BSB. Figure 4 corresponds to the 1H NMR of solvolysis samples catalyzed by 3005 ppm of TTiP at 170°C from the initial time (lower) until reaching equilibrium (upper) obtained according to Example 1. The chemical shift in ppm is represented on the abscissa axis of the 1H NMR spectrum. Figure 5 shows the chromatograms obtained by gel permeation chromatography (GPC) of the solvolysis samples at 170ºC at initial time (medium) and after hours of reaction (top) obtained according to Example 1. The abscissa axis of the chromatogram represents the retention time in minutes, while the ordinate axis represents the refractive index (RI) expressed in mV. Figure 6 shows the molecular weight distribution of petrochemical PBS, partially renewable PBS and repolymerized PBS, obtained according to Example 1, so the abscissa axis represents the logarithm of the molecular weight (M) expressed in (g / mol) , while the ordinate axis represents a normalized mass fraction distribution. Figure 7.- Corresponds to the 1H NMR of solvolysis samples catalyzed by 3005 ppm of TTiP at 130ºC, 170ºC and 200°C at initial time (lower) until reaching equilibrium (upper), obtained according to Example 2. On the abscissa axis of the 1H NMR spectrum, the chemical shift in ppm is represented. Figure 8 shows the GPC chromatograms of the solvolysis samples at 130ºC, 170ºC and 200ºC, obtained according to Example 2. The logarithm of the molecular weight expressed in (g / mol) is represented on the abscissa axis of the chromatogram, while a normalized mass fraction distribution is represented on the ordinate axis. Figure 9.- Corresponds to the 1H NMR of solvolysis samples catalyzed by titanium tetrabutoxide (IV) at 170°C at initial time (lower) and final time (upper), obtained according to Example 3. The chemical shift in ppm is represented on the abscissa axis of the 1H NMR spectrum. Figure 10.- Corresponds to the 1H NMR of solvolysis samples catalyzed by zirconium tetrabutoxide (IV) at 170°C at initial time (lower) and final time (upper), obtained according to Example 3. The chemical shift in ppm is represented on the abscissa axis of the 1H NMR spectrum. Figure 11.- Corresponds to the 1H NMR of solvolysis samples catalyzed by zirconium tetraisopropoxide (IV) at 170°C at initial time (lower) and final time (upper), obtained according to Example 3. The chemical shift in ppm is represented on the abscissa axis of the 1H NMR spectrum. Figure 12 shows the GPC chromatograms of the solvolysis samples with different catalysts based on Ti(IV) and Zr(IV) metals, obtained according to Example 3. The logarithm of the molecular weight expressed in (g / mol) is represented on the abscissa axis of the chromatogram, while a normalized mass fraction distribution is represented on the ordinate axis. Figure 13 shows the GPC chromatograms of the solvolysis samples with TTiP in amounts of 1503, 3005 and 4508 ppm, obtained according to Example 4. The logarithm of the molecular weight expressed in (g / mol) is represented on the abscissa axis of the chromatogram, while a normalized mass fraction distribution is represented on the ordinate axis. DETAILED EXPOSURE OF MODES OF REALIZATION Figure 1 shows a diagram, according to the preferred embodiment of the invention, of the chemical recycling process for PBS, detailing the following: In the first stage (depolymerization), a solvolysis reaction is performed on the dry PBS powder to be recycled. In a preferred embodiment of the invention, this solvolysis reaction is carried out in an experimental setup consisting of a temperature-controlled and stirring glass reactor to which a cooling column is connected. PBS is introduced along with 1,4-BDO as a nucleophile, using TTiP as a catalyst. As a result of this reaction, the compound BSB is obtained, according to the following reaction scheme (1): Thus, as an example, in a laboratory setup, a glass reactor is used where ground PBS and 1,4-BDO are introduced in a 1:5 weight ratio, respectively. The setup is sealed with the cooling column, and the system is purged with a stream of inert nitrogen gas. Then, 500 ppm of Ti4+ in the form of TTiP (3005 ppm equivalent) relative to the biopolymer is added, and a heating ramp is carried out until a final temperature of, preferably, 170°C is reached. Once this temperature is reached, the resulting mixture is stirred for 4 hours. It is then cooled to room temperature and extracted with CHCl3, yielding an organic phase (OP) and an aqueous phase (AFP). These two phases are then separated by washing with brine. After drying with anhydrous MgSO4 and concentrating the organic phase (OP) at reduced pressures using a rotary evaporator, the desired product, BSB, is obtained as a white, viscous liquid with an approximate yield of 90% and without further purification. Finally, the excess brine used for washing is filtered, yielding a white solid on one hand and a liquid on the other. The liquid is then distilled to recover the BDO. To confirm the presence of BSB as the final product of depolymerization, a characterization is performed using 1H-NMR in deuterated chloroform, obtaining the graph represented in Figure 2. Next, and consecutively, after depolymerization, the repolymerization stage is carried out. Thus, in the second stage, the PBS is repolymerized from BSB. It should be noted that, in a preferred embodiment of the invention, the same catalyst used in the depolymerization stage is employed in the repolymerization stage. For this purpose, the BSB is introduced into a jacketed, five-necked glass reactor (2) connected to a Huber Pilot ONE CC-304B immersion bath with system temperature control. A mechanical stirrer and an elbow (3) connected to a cooling column, with a distillation elbow (4) and vacuum vent (5), are connected to the reactor (2). Figure 3 shows a reaction system used during the second stage. At the beginning of this repolymerization stage, the reactor (2) is closed and the system is purged with a stream of inert gas, preferably nitrogen, at a flow rate of 0.1 L / min for 1 hour, while the BSB is stirred at low revolutions (i.e., at a minimum of 25 rpm, preferably 30 rpm) to remove any possible moisture from the system. Subsequently, a heating ramp is performed from ambient temperature to a temperature equal to or greater than 100°C. Once this temperature is reached, between 1803 and 2404 ppm of catalyst are added, the system is closed, and the inert gas flow is stopped. The system is then brought under high vacuum (i.e., to a pressure below 2 mbar) and the heating ramp is continued to a reaction temperature of 230°C. This process is carried out at an approximate heating rate of 3°C / min, taking care to avoid excessive bubble formation and sublimation of the oligomer. Once the reaction temperature is reached, the stirring is increased to 75 rpm for 4 hours or until the mixture reaches a viscosity increase that indicates the formation of high molecular weight PBS, i.e., with a molecular weight between 50,000 and 250,000 g / mol. Finally, the reactor system is allowed to cool to room temperature and the solidified PBS obtained (corresponding to a whitish solid mass) is removed from the reactor, ground in an analytical mill, washed and dried under the same conditions as the PBS used at the beginning of the detailed procedure. After repolymerization, the recycled PBS is characterized and compared with the same biopolymer before the chemical recycling procedure to which it was subjected. To illustrate this, the determination of the number (Mn) and weight (Mw) molecular weight values ​​of recycled PBS and the original PBS used, in this case partially obtained from renewable sources, is shown. These values, obtained by 1H NMR and GPC, are directly related to the length of the biopolyester chains. According to the data shown in Table 1, the molecular weight of the PBS obtained by the procedure of the invention (recycled PBS) is of the same order as the initial commercial product, with Mw values ​​in a range of 100,000-150,000 g / mol and ΐ between 2 and 3. Therefore, it can be concluded that the characteristics of the PBS recycled by the described procedure are similar to the original. Table 1. Average number molecular weight (Mn), weight molecular weight (Mw) and dispersion (ΐ) of original and recycled polybutylene succinate (PBS). Values ​​determined by proton nuclear magnetic resonance (1H-NMR) and gel permeation chromatography (GPC). Examples of preferred embodiments of the invention are then detailed. The following examples explore various factors that can affect depolymerization and repolymerization processes. First, the study aims to examine the effects of temperature, catalyst type, and optimal catalyst concentration on the depolymerization process. Second, it seeks to observe how the depolymerization product, as a starting material, affects the repolymerization process. To this end, in the first example (Example 1), the complete depolymerization process of PBS is carried out using catalyzed solvolysis, followed by solventless repolymerization. The goal is to analyze the feasibility and efficiency of the complete chemical recycling process (depolymerization + repolymerization) along with the properties of the repolymerized PBS, specifically its molecular weight. In a second example (Example 2), the depolymerization process is repeated, but with a different reaction temperature. This study aims to analyze the influence of temperature on the depolymerization kinetics and the characteristics of the resulting products. In a third example (Example 3), the depolymerization process is repeated using different types of Ti and Zr catalysts and different concentrations, seeking to compare the efficiency between different catalysts in the depolymerization of PBS. In a fourth example (Example 4), the depolymerization process described in Example 3 is repeated using the optimum catalyst, but varying its concentration to analyze the effect on the yield and rate of depolymerization, as well as the properties of the final product. And a final example (Example 5), where the depolymerization process is repeated at a less favored temperature, followed by the repolymerization process described in the first example (Example 1) to evaluate the effectiveness of the depolymerization process at a lower temperature, observing whether it is possible to obtain a repolymerized PBS with good properties. Example 1: The depolymerization process was carried out by solvolysis of PBS in a temperature-controlled reactor with stirring and a cooling system. Polyester was introduced along with 1,4-BDO in a 1:5 weight ratio and 3005 ppm equivalents of TTiP catalyst. The system was heated to 170°C, and the depolymerization product was obtained after extraction. Samples were taken at different times. Each sample was analyzed by 1H NMR to monitor the reaction's progress over time. 1H NMR spectra were recorded on a Bruker AMX-300 spectrometer at 25°C, operating at 300.1 MHz. Samples were dissolved at concentrations of 10–20 mg / mL CDCl3, using tetramethylsilane (TMS) as an internal reference. A total of 128 scans were recorded with 32,000 data points and 2-s waiting times. Spectra were processed using Bruker WINNMR 1D software. Molecular weight distributions of the final product were obtained by GPC using a Waters chromatograph (Milford, MA, USA) equipped with refractive index (RI) and ultraviolet (UV) detectors. For this purpose, 2-3 mg of biopolyester were dissolved in 1 mL of chloroform (CHCl3, HPLC grade, Fisher Scientific SL, Alcobendas, Spain) and filtered with a 0.22 µm polytetrafluoroethylene (PTFE) filter (Phenomenex, Alcobendas, Spain).Next, 100 µL of this solution were injected into the system and eluted with CHCl3 at a flow rate of 0.5 mL·min-1. Linear polystyrene columns (7.8 x 300 mm, 103-104 Å pore size, Waters Cromatografía, SA, Cerdañola del Vallés, Spain) packed with crosslinked polystyrene (PS) and protected with a pre-column were used. PS standards with narrow molecular weight distributions were used to generate the calibration curve. The repolymerization process was carried out by polycondensation of the BSB and oligomers in the absence of solvents, avoiding the prior esterification step, unlike the methods described in the compiled literature. Only the depolymerization product obtained in the previous step was introduced into a reactor with temperature control, agitation, a cooling system (collector), and high vacuum. This process consists of the following steps: - After the introduction of the depolymerization product, the reactor was shut down, stirring was started under moderate conditions (e.g., 25 rpm) and a purge was carried out with an inert nitrogen gas stream to remove / reduce moisture from the system. - Heating the reactor to a temperature above 100°C. - Addition of between 1803 and 2404 ppm of the TTiP catalyst, cessation of the inert gas flow and closure of the system for vacuum application. - Applying a vacuum to the reactor up to a pressure of at least 200 mbar. - Heating the reactor to a temperature of at least 230°C, with the application of a moderate heating ramp of 1 to 10°C / min, taking care to avoid excessive bubble formation. - Gradually decrease the pressure until it reaches less than 1 mbar, preventing the sublimation of BSB and oligomers. - Increase agitation, for example, to 75 rpm, and maintain the reaction until the mixture reaches a maximum viscosity increase, indicating the formation of high molecular weight PBS. - Cooling to room temperature, resulting in the solidification of the PBS. The repolymerized polymer was analyzed by 1H-NMR and Mn, Mw, ΐ values ​​and the molecular weight distribution were obtained by GPC. In the resonance spectra, the peaks of the depolymerization product are defined at the following shifts: 1.70-1.93 (8H, 1.76 (tt, J = 7.5, 6.9 Hz) , 1.86 (tt, J = 7.5, 7.2 Hz) ), 2.75 (4H, t, J = 7.4 Hz) , 3.34 (4H, t, J = 6.9 Hz) , 4.16 (4H, t, J = 7.2 Hz) . According to the resonance spectra of the samples taken during the course of the reaction, no differences are observed between the sample at the initial time and the last sample taken, so it is understood that at the initial time equilibrium has already been reached and the reaction completed (Fig.4). According to the chromatograms (Fig. 5), both at the initial and final times during the reaction, signals were observed not only for the compound bis(4-hydroxybutyl)succinate (BSB), but also for the pentaester dimer of 1,4-butanediol and succinic acid (BSBSB) and the heptaester trimer of succinic acid and 1,4-butanediol (BSBSBSB) in different proportions, with the majority signal corresponding to BSB (longer retention times, lower chain lengths and molecular weights). The ratio between the peaks was not influenced by the reaction time. The values ​​of Mn, Mw, and ΐ were determined by GPC, and the molecular weight distribution (Fig. 6) of the repolymerized biopolyester was obtained and compared with commercial samples of partially renewable, petrochemical-based PBS. The molecular weight of the repolymerized polyester was similar to that of the commercial petrochemical polymer. Table 2. Average number molecular weight (Mn), weight molecular weight (Mw) and dispersion (Δ) of repolymerized polybutylene succinate (PBS) alongside commercial samples of partially renewable and petrochemical PBS. Values ​​obtained by gel permeation chromatography (GPC). Example 2: The depolymerization process described above in Example 1 was carried out at different reaction temperatures, and samples were taken at different times. Each sample was analyzed by 1H NMR, and the final depolymerization products were analyzed by GPC. In the resonance spectra of the samples taken during the reaction tests at 130°C, changes in the intensity of the triplet peaks at 3.6 ppm are observed, from the start of the reaction until the signal remains constant and reaches depolymerization equilibrium (Fig. 7). However, in the test at 200°C, no changes are observed between the sample at the initial time and the last sample taken, so it is understood that at the initial time (time required for sample dissolution 10-15 min) equilibrium has already been reached and the reaction completed. According to the chromatograms for the tests at different temperatures (Fig. 8), the signals corresponding to BSB are observed in the highest proportion, followed by the BSBSB dimer and, in a smaller proportion, the BSBSBSB trimer. Thus, the depolymerization product at 130°C has the highest proportion of dimer and trimer compared to the other tests, where the presence of longer chains could affect the subsequent repolymerization process. Similar results were observed in the tests at 170°C and 200°C in terms of the proportion of BSB compared to the dimer. Considering the resonance spectra along with the chromatograms, the tests at 170°C and 200°C yielded very similar results at the initial and final times. Therefore, the depolymerization reaction temperature was set at 170°C, instead of 200°C, due to the resulting energy savings from the lower temperature. Example 3: The depolymerization process described above in Example 1 was carried out using different catalysts based on titanium(IV), Ti(IV) tetraisopropoxide and Ti(IV) tetrabutoxide (Ti(iOPr)4 and Ti(OBut)4), and zirconium(IV), Zr(IV) tetraisopropoxide and Zr(IV) tetrabutoxide (Zr(iOPr)4 and Zr(OBut)4), at different concentrations, and samples were taken at different times. Each sample was analyzed by 1H-NMR and the depolymerization products by GPC. Table 3. Range of Ti(IV) tetraisopropoxide, Ti(IV) tetrabutoxide, Zr(IV) tetraisopropoxide and Zr(IV) tetrabutoxide catalysts (Ti(iOPr)4), (Ti(OBut)4), (Zr(iOPr)4) and (Zr(OBut)4) used at different concentrations in the depolymerization process. In the resonance spectra obtained at initial and final time for the different catalysts (Ti(iOPr)₄, Ti(OBut)₄, Zr(OBut)₄, and Zr(iOPr)₄) used in the depolymerization tests (Figs. 4, 9, 10, and 11, respectively), we can observe that for the tests carried out with titanium(IV) catalysts, the reaction reached equilibrium at the initial time. However, for those tests in which zirconium(IV) catalysts were used, a difference in the intensity of the triplet signal at 3.6 ppm at the initial time is noticeable, indicating that the depolymerization process has not been completed and that a longer time is required to reach equilibrium. The chromatograms (Fig. 12) show the differences between the use of titanium(IV) and zirconium(IV) catalysts, with titanium catalysts resulting in a higher proportion of BSB compared to dimer and trimer. However, the opposite is true for zirconium catalysts, with a higher proportion of dimer compared to BSB and trimer. Since titanium catalysts provide a reaction product with a higher proportion of BSB, the depolymerization process performs better than in the case of zirconium catalysts. Example 4: The depolymerization process described above in Example 1 was carried out at different concentrations of TTiP catalyst (mg TTiP / kg PBS), and samples were taken at different times. Each sample was analyzed by 1H NMR, and the depolymerization products were analyzed by GPC. The resonance spectra of samples taken during the tests at 1503, 3005, and 4508 ppm of the TTiP catalyst showed no appreciable changes between tests. However, as observed in the chromatograms (Fig. 13), the tests using 3005 ppm of catalyst had a lower proportion of dimer than those conducted with 1503 and 4508 ppm. Therefore, it was decided to use the catalyst concentration that resulted in the lowest proportion of dimer. Example 5: The depolymerization process described in Example 1 was carried out at a temperature of 130°C, followed by the repolymerization process described above. A repolymerized biopolyester was obtained with a molecular weight significantly lower than that achieved under the conditions described in Example 1. Table 4. Average number molecular weight (Mn), weight molecular weight (Mw) and dispersion (ΐ) of repolymerized polybutylene succinate (PBS) starting from a depolymerization product with a higher proportion of dimer and trimer. Values ​​obtained by gel permeation chromatography (GPC). Conclusions on Depolymerization and Repolymerization of Renewable PBS The complete chemical recycling process of renewable PBS by catalyzed solvolysis depolymerization followed by solventless repolymerization has been shown to be technically and chemically viable according to the steps detailed in the present invention. From the proposed invention procedure, it is clearly demonstrated that a repolymerized biopolyester was obtained with properties comparable to those of commercial PBS, particularly in terms of molecular weight and dispersion (Table 2), which validates this chemical recycling procedure as a sustainable and efficient alternative for the recycling of polyester materials. Among the most determining factors in the depolymerization process is the reaction temperature, which directly influences the kinetics and composition of the product. At high temperatures (between 170°C and 200°C), the reaction reaches equilibrium quickly, generating a greater proportion of the BSB monomer, essential for subsequent effective repolymerization. Conversely, at lower temperatures (130°C), the reaction is slower and a greater accumulation of oligomers (dimers and trimers) is observed, resulting in a repolymerized PBS with a lower molecular weight and, therefore, processability and properties unsuitable for food packaging. Additionally, the choice of 170°C as the optimum temperature represents a suitable compromise between performance, energy efficiency, and process sustainability. Furthermore, the type of catalyst used proved to be another key parameter. Titanium IV-based catalysts, namely Ti(iOPr) and Ti(OBut)₂, exhibited significantly higher performance than their zirconium IV counterparts, favoring the formation of the BSB monomer and minimizing the presence of oligomers (Fig. 12). In contrast, zirconium IV catalysts promoted the formation of dimers, a factor that has been shown to hinder subsequent repolymerization. Regarding the catalyst concentration in the depolymerization process, a dose of 3005 ppm of Ti(iOPr) was determined to be the most suitable. This concentration promotes a higher proportion of BSB without unnecessarily increasing the presence of oligomers or compromising the economic balance of the process. Higher concentrations (4508 ppm) did not show any additional improvements. Finally, it is confirmed that the success of repolymerization is directly conditioned by the composition of the previous depolymerization product. A high BSB monomer content is critical for achieving high molecular weights in the repolymerized PBS. Therefore, controlling the depolymerization conditions, especially temperature and catalyst type, is fundamental to ensuring the efficiency and quality of the chemical recycling process.

Claims

1- A chemical recycling process for polybutylene succinate comprising a depolymerization process and a repolymerization process, carried out consecutively, wherein the depolymerization comprises the following steps: Solvolysis reaction of PBS in a stirred reactor, where the polybutylene succinate (PBS) is introduced together with 1,4-butanediol (1,4-BDO) and heated between 130°C and 200°C, so that bis(4-hydroxybutyl)succinate (BSB) is obtained as a product. Whereas in the repolymerization, the self-polycondensation of the BSB previously obtained during the depolymerization is carried out in the absence of solvents, avoiding a prior esterification step and introducing only said BSB into a reactor, such that the repolymerization comprises the following steps: - Introduction into the reactor of the BSB obtained in the solvolysis reaction, closing of the reactor, initial stirring a, at least,- Heating the reactor to a temperature of 100°C or higher. - Addition of titanium(IV) tetraisopropoxide (TTiP) catalyst to the reactor at a concentration between 1803 and 2404 ppm, forming a mixture, and cessation of the inert gas flow. - Application of a vacuum to the reactor to a pressure of at least 200 mbar. - Heating the reactor to a temperature of at least 230°C, with a heating ramp of 0.5 to 25°C / min. - Gradually decreasing the reactor pressure to less than 1 mbar. - Increasing the initial stirring speed to at least 75 rpm and maintaining the mixture in the reactor until the formation of PBS with a weight molecular weight (Mw) in the range of 50,000–250,000 g / mol. - Cooling to room temperature,solidification of the PBS occurs. Where the resulting PBS biopolyester has a Mw between 50,000 and 250,000 g / mol. 2- Chemical recycling process of polybutylene succinate, according to claim 1, characterized in that in the solvolysis reaction the PBS is introduced together with 1,4-BDO in the reactor with a TTiP catalyst. 3- Chemical recycling process of polybutylene succinate, according to claim 2, characterized in that the concentration of TTiP catalyst in the depolymerization is 3005 ppm.

4. A chemical recycling process for polybutylene succinate, according to claim 1, characterized in that prior to depolymerization, the PBS is ground, washed with water and / or ethanol under agitation for at least four hours at 55°C, and vacuum dried at a temperature of at least 60°C for at least eight hours.

5. A chemical recycling process for polybutylene succinate, according to claim 1.characterized in that the inert gas stream is nitrogen or argon. 6- Chemical recycling process of polybutylene succinate, according to claim 1, characterized in that a pressure of less than 2 mbar is applied in the reactor during the vacuum application in the repolymerization. 7- Chemical recycling process of polybutylene succinate, according to claim 1, characterized in that the solidified PBS obtained after cooling is milled in an analytical mill, washed, and dried. 8- Chemical recycling process of polybutylene succinate, according to claim 7, characterized in that the washing of the solidified PBS is carried out with water and / or ethanol under agitation for at least four hours at 55°C and vacuum drying at a temperature of at least 60°C for at least eight hours. 9- PBS biopolyester obtained using the process according to any of the preceding claims,characterized in that it has an Mw between 50,000 and 250,000 g / mol. 10- Biopolyester PBS, according to claim 9, characterized in that it has a polydispersity index (ΐ) between 2 and 3.

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